1. Field of the Invention
The present invention relates to a thermal printing mechanism, a thermal printer having the thermal printing mechanism, and a thermal printing method.
2. Description of the Related Art
One conventional thermal printer carries out printing (recording) by bringing a heating element whose temperature has been raised into contact with a thermochromic recording medium, causing the portion of thermochromic recording medium coming into contact with the heating element to develop color. Such a thermal printer energizes the heating element to raise the temperature.
Another thermal printer carries out so-called multigradation printing (multigradation recording) in which the period during which energization of the heating element is adjusted to adjust the color density of the recording medium. A thermal printer that carries out such multigradation printing, for example, extends the period that the heating element is energized as the density of a pixel to be printed increases.
Specifically, a conventional technique is known, according to which the total time required for printing one pixel is divided into energization time units represented as powers of 2, based on binary notation, and the heating element is energized for a given energization time unit that has been selected from among the energization time units as a energization time equivalent to the amount of time that the heating element is to be energized. Thus, this technique weights energization times so that the period during which the heating element is energized increases as the density of the pixel to be printed increases.
In this manner, according to the conventional method of carrying out multigradation printing by weighting energization times, energization time units are sorted in descending order or ascending order, and a heating element is energized during a given energization time unit that is selected from among the sorted energization time units based on the density of a pixel to be printed.
Specifically, another conventional technique is known, according to which, for example, the median point of the total time period is matched to the median of the number of heating pulses necessary for achieving printing densities (recording densities) respective to each pixel so that energization equivalent to the number of the necessary heating pulses are carried out consecutively (see, e.g., Japanese Laid-Open Patent Publication No. H5-42706).
Still another conventional technique is known, according to which, for example, when the length of the heating/energization period is changed according to the density of the pixel to be printed, the start of a energization period within the total time period (printing cycle time for one pixel) is controlled so that the interval between the median points of energization periods of consecutive printing cycles becomes substantially equal to the printing cycle time for one pixel (see, e.g., Japanese Laid-Open Patent Publication No. H5-278253).
Yet another conventional technique is known, according to which, for example, when the density gradation of the pixel to be printed is N and the heating time for achieving a coloring area with the maximum density gradation is T, the heating period for achieving coloring areas corresponding to each density gradation is determined to be integral multiple T/(N−1) and such heating periods corresponding to density gradations are arranged as a first half portion and a latter half portion that are substantially equal to each other with respect to the point of elapse of T/2 from the start of the heating period T (see, e.g., Japanese Laid-Open Patent Publication No. S62-260476).
When energization is carried out by weighting energization times, as in the case of binary-based energization, the energization according to the energization times sorted in descending or ascending order may result in the appearance of a white line at a point of gradation change, depending on the printing speed at the time that the energization is carried out. To deal with this problem, the inventor has devised a technique of further dividing each of the already divided energization times into two and carrying out energization in a pattern such that in the total time required for recording one pixel, energization time units for energization appear as a first half portion and a latter half portion that are substantially symmetrical with respect to the median time point of the total time period, to make inconspicuous a white or black line at a point of density change.
If the printing speed is low, however, the printing pattern of the previous line has less influence on the next line. Specifically, a rise in the temperature of a heating element resulting from the energization for the printing of the previous line exerts less influence on the printing of the subsequent line. This leads to a problem in that if the number of divisions of energization times is increased by a further division of each divided energization time into two, etc., in the event of a low printing speed, a disadvantage of a decline in the extent to which the temperature of the heating element rises consequent to the energization, that is, a decline in heat efficiency becomes larger than the effect of making inconspicuous a white or black line at a point of density change.